This invention relates to a device for limiting current, and more particularly, to a medical current limiting circuit. The device is useful for protecting medical electrical apparatus and patients being monitored by electrocardiographic equipment from possible current flow caused by the patient's contact with an outside current source or by a failure in the amplifier section of the equipment.
In the past, a variety of devices have been utilized to limit current flow. The classical method involves placing a large resistor in series with the current source and the load. This method is undesireable for medical diagnostic applications because large resistors add "Johnson noise" which interferes with subsequent signal analysis. Phase shifts, frequency distortions, amplifier current noise and loss of common-mode rejection capabilities also result from the use of a large series resistance.
One prior art device utilizes a pair of field effect transistors (FET), connected in series with a small resistance in between to limit current. However, this device is ineffective at limiting current to desired medically safe levels, for example under 20 micro-amps, without the use of large resistors. Another problem with this device is that, upon sensing an overload, it limits current to a fixed maximum value which is independent of the applied voltage, as opposed to dropping the current to a very low idle level, for example under 4 micro-amps.
Another current limiter device utilizes a pair of exterior n-type FETs (n-FET) in series with a p-type FET (p-FET) placed in between. This device is used for logic in computers. The p-FET is controlled by an external voltage connected to its gate, and thus acts as a voltage controlled variable resistor. The requirement of an external control voltage renders this device unusable for fail-safe applications such as patient protection.
Yet another device has an n-FET and a p-FET connected in series. Due to polarity changes in alternating current (a.c.) circuits, or voltage variations in a pulsating direct current (d.c.) circuit, a device such as this repeatedly switches on and off and must be re-educated at each successive cycle as to the existence of excess current. Thus, its average current is unacceptably high and signal transients are introduced into the system.
Despite the need for a current limiting device which is usable for patient protection in electro-medical diagnostic and therapeutic applications, and which overcomes prior art problems and limitations, none insofar as is known has been proposed or developed.
Accordingly, it is an object of the present invention to provide a device that is an inexpensive and compact fail-safe current limiter which is usable with modern medical electrodes and associated diagnostic and therapeutic apparatus to protect a patient from accidental electrical shock. It is a further object of this invention to provide a device which does not introduce deleterious Johnson noise, phase shifts, frequency distortions or amplifier current noise into the signal analysis process, and which does not cause a loss of common-mode rejection capabilities.
Another object of this invention is to provide a device which shuts off current flow to a very low idle current, for example less than 20 micro-amps, upon detection of a current overload and which is self-resetting. Another object of this invention is to provide a device which latches its idle current to ride out successive cycles of a.c. polarity change or pulsating d.c. voltage variations without repeatedly switching on and off.